Causes of Climate Change Over the Past 1000 Years

Rcjohnsen rcjohnsen at aol.com
Wed Aug 9 18:06:27 EST 2000


Atmospheric Science   Sci 289:270  7-14-00

Causes of Climate Change Over the Past 1000 Years 

Thomas J. Crowley 
Recent reconstructions of Northern Hemisphere temperatures and climate forcing
over the past 1000 years allow the warming of the 20th century to be placed
within a historical context and various mechanisms of climate change to be
tested. Comparisons of observations with simulations from an energy balance
climate model indicate that as much as 41 to 64% of preanthropogenic (pre-1850)
decadal-scale temperature variations was due to changes in solar irradiance and
volcanism. Removal of the forced response from reconstructed temperature time
series yields residuals that show similar variability to those of control runs
of coupled models, thereby lending support to the models' value as estimates of
low-frequency variability in the climate system. Removal of all forcing except
greenhouse gases from the ~1000-year time series results in a residual with a
very large late-20th-century warming that closely agrees with the response
predicted from greenhouse gas forcing. The combination of a unique level of
temperature increase in the late 20th century and improved constraints on the
role of natural variability provides further evidence that the greenhouse
effect has already established itself above the level of natural variability in
the climate system. A 21st-century global warming projection far exceeds the
natural variability of the past 1000 years and is greater than the best
estimate of global temperature change for the last interglacial. 






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